The invention relates to a method for preparing mechanically stable molecular sieve catalyst compositions possessing a titania binder of low acidity.
The catalysts of the invention possess significant physical strength and will resist abrasion and fracture during loading and transit. These catalysts easily withstand loads in a catalyst bed due to fluid pressure drop and weight of superimposed catalyst. They also have superior interior cohesion allowing for calcination and regeneration without catalyst particle breakup.
Molecular sieves for use as catalyst components herein include zeolites, aluminophosphates, silicoaluminophosphates, layered and pillared-layered materials and other catalytically active crystalline materials.
The term "zeolite" as used herein designates the class of porous crystalline silicates, which contain silicon and oxygen atoms as major components. Other framework components can be present in usually less than about 14 mole%, and often less than about 4%. These components include aluminum, gallium, iron, boron, etc., and combinations thereof. The crystalline aluminosilicates constitute an especially well known type of zeolite.
It is well known that extrusion is one way of obtaining a molecular sieve-containing material which has a high degree of strength for various applications, both catalytic and noncatalytic. Some molecular sieves, e.g., aluminosilicate zeolites, have long been used as catalysts for a wide variety of organic conversion processes. In general, the molecular sieve is incorporated with a matrix, or binder material in order to impart mechanical stability hereto. The most commonly used matrix materials have included alumina and/or clays since these materials are fairly easy to extrude and provide extrudates of good physical strength for many applications.
It has come to be recognized that low acidity refractory oxides, such as titania, zirconia and silica, are desirable matrix materials and that they possess advantages over alumina for some catalytic reactions. In this connection, U.S. Pat. No. 4,013,732 specifically discloses ZSM-5 with a silica matrix and U.S. Pat. Nos. 3,843,741 and 3,702,886 broadly disclose the use of ZSM-5 with a silica matrix.
U.S. Pat. No. 4,582,815 describes a method for preparing silica-rich solids said to possess improved crush strength compared to that of known silica-bound materials. The method comprises mixing silica-rich solids, preferably a mixture of silica with a zeolite such as ZSM-4 (Omega), ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-48, Beta, X, Y, L, ferrierite, mordenite, dachiardite, clinoptilolite, offretite, erionite, gmelinite, chabazite, etc., with water and an alkali metal base such as sodium hydroxide or a basic salt such as an alkali-metal carbonate, borate, phosphate, silicate, etc., as an extrusion aid followed by mulling, extruding and subsequently drying the extrudate. It is thought that substitution of alkali metal for hydrogen in the silanol groups on the surfaces of siliceous materials such as the foregoing zeolites is responsible for their improved crush strength. The resulting extrudate is said to possess superior crush strength and sufficient integrity to withstand treatments with acids so that is possible to steam, acid extract or calcine them. To avoid trapping the alkali metal of the extrusion aid in the extrudate, the alkali metal is ordinarily removed by exchange under acidic conditions using dilute nitric acid in 1M ammonium nitrate solution.
Silica-bound zeolite catalysts prepared in accordance with the method described in U.S. Pat. No. 4,582,815 are indicated to be useful in hydrocarbon conversions such as hydrocracking, isomerization, hydrogenation, dehydrogenation, polymerization, reforming, catalytic cracking and catalytic hydrocracking.
U.S. Pat. No. 5,053,374 teaches another method for preparing zeolite catalysts bound with low acidity refractory oxide, e.g., silica. The method involves providing a substantially homogeneous mixture of zeolite, water and low acidity binder at least partly in colloidal form. The mixture is free of added alkali metal base and/or basic salt. The mixture is extruded, dried and calcined to provide the catalyst.
Another method for the binding of zeolites with zirconia to produce catalysts with commercially acceptable physical strength is shown in U.S. Pat. No. 5,182,242 which teaches the use of polymeric, hydroxy or alkoxy-bridged zirconia solutions, such as ammonium zirconium carbonate in conjunction with zirconium oxide.